She’d been living in Montana for at least 25 years, along with her daughter Maggie and her grandchildren. She became a U.S. citizen in 2005.

Kidder was born in Yellowknife, N.W.T. on Oct. 17, 1948, one of five children. (Her sister, Annie Kidder, resides in Toronto and works for Canadian registered charity People for Education. A brother, Michael Kidder, lives in Vancouver.) She travelled a lot as a child because of her father’s job, and spent a few years as a child in Labrador City, Newfoundland and Labrador. She went to multiple schools and eventually graduated from Toronto’s Havergal College in 1966.

WATCH: Margot Kidder, known as Lois Lane in ‘Superman,’ died at 69

She got a taste of showbiz when her mother took her to New York when she was 12. It was a sign for the impressionable Kidder.

“We didn’t have movies in this little mining town. When I was 12 my mom took me to New York and I saw Bye Bye Birdie, with people singing and dancing, and that was it,” she said to The Guardian in 2005. I knew I had to go far away. I was clueless, but I OK.”

She catapulted to fame for her performance as a journalist and Superman’s love interest, Lois Lane, in 1978’s Superman. She went on to star in Superman II, III and IV.

Kidder became an advocate for mental health issues after speaking out about living with bipolar disorder. She had an infamous breakdown on the streets of Los Angeles in 1996.

She credited a homeless man with showing compassion for her and saving her life during her “big, public flipout.”

“We are all … a breath away from mental illness, homelessness, all of these things we tend to so look down on,” Kidder said in 2006. “We are all one human family and we really have to take care of each other.”

She also became a political activist in recent years and was among a group of environmentalists to be arrested outside the White House in 2011 during a protest against TransCanada’s Keystone XL pipeline.

Kidder had settled in Montana to live in a “culture-free zone” away from the spotlight.

Hollywood personalities took to Twitter to share their condolences at her passing.

Superman was a superhero blockbuster two decades before comic book movies became the norm at the top of the box office.

Both Kidder and co-star Reeve, who played Superman, were relative unknowns when they got their leading parts, and neither saw many major roles afterward. Reeve died in 2004 after becoming paralyzed in a horse-riding accident.

Kidder also appeared in 1975’s The Great Waldo Pepper with Robert Redford and 1978’s The Amityville Horror.

She continued to work in TV and film despite her struggles, and won a daytime Emmy in 2015 for outstanding performance in a children’s or preschool series for R.L. Stine’s The Haunting Hour.

She appeared in stage productions such as The Vagina Monologues and Love, Loss and What I Wore.

Kidder was married and divorced three times and was also famously linked to former prime minister Pierre Elliott Trudeau. Kidder was credited with influencing Trudeau’s decision to launch a global peace initiative during his final months in office, according to the 2009 biography Just Watch Me: The Life of Pierre Elliott Trudeau 1968-2000.

Annie said that Kidder’s health had not been great recently, but any problems were mostly due to aging. An autopsy is scheduled to be performed. As of this writing, there is no additional funeral information.